Where
is the oil likely to spread?

Dr. Richard Garvine at the University of Delaware College of Earth, Ocean, and Environment
provides this analysis of the Delaware
River and Bay's circulation:

"From
our group’s
study of the circulation and mixing of water in the Delaware Estuary (the Delaware
Bay and the tidal portion of the Delaware River, which extends to the falls
at Trenton, New Jersey) over the past two decades, we have assembled a coherent
view of how estuarine circulation works in general.

The dominant source of current in the estuary is the tidal period (12.4 hr)
rise and fall of the water level impressed on the estuary’s mouth by
the tidal variations on the adjacent continental shelf. Typical tidal currents
in the estuary are about 0.8 meters/second or about 1.5 mph. The mechanism
creating the next rank of tidal current is the horizontal difference in water
density between the fresh water in the Delaware River and the heavier water
seaward in the estuary where shelf water has intruded at depth and mixed upward
with seaward flowing fresh water. This force creates a strong conveyor belt-like
circulation pattern in the vertical with the lighter water passing through
seaward in the upper half of the water depth and landward in the bottom half.
A similar circulation is established in a room with a wood stove in operation.
Hotter (less dense) air rises and travels upward along the ceiling while (more
dense) air sinks and travels along the floor inward toward the stove. Typical
current speeds for density-driven circulation in the Delaware estuary are about
0.05 to 0.10 m/s or about 0.1 to 0.2 mph.

Wind produces current, also. Wind from the northwest induces a seaward flow
for the upper 2/3 of the water depth and a landward flow in the lower 1/3 with
typical speeds the same as the density driven flow. The directions reverse
for wind from the southeast. But there is a special twist to the wind-forced
current. Strong winds from the southwest impact the estuary little, as they
are crosswinds there, while on the shelf nearby they produce coastal upwelling
and falling sea level. This is imposed on the estuary mouth causing falling
water levels there, too. In the reverse with winds from the northeast (as in
a ‘northeaster’), sea-level rise results, often with coastal flooding.
Currents in the estuary then reflect this water-level change by flowing landward
in northeasters and seaward in southwesterly winds. The speeds from this action
are a fraction of the locally driven winds discussed above.

In the estuary, the strong tidal currents themselves with daily and twice-daily
periods produce a steady or very slowly varying current called the tidal residual.
This flow is weaker at about 0.05 mph, but makes up in persistence by its long
term of activity in transporting materials, including petroleum. Last in intensity
is the current driven by the water added to the estuary by its tributaries,
the largest being the Delaware River. This flow drifts seaward at all depths
much like water running downhill in a rain gutter. Typical speeds are 0.005
mph or about 0.25 miles/day.

Averaging over the tides, we find under most conditions a net seaward flow,
especially in the upper half of the depth. Most of the oil from a spill will
be in this part of the water column, and we can expect typical current speeds
of 0.1-0.2 mph or about 2-5 miles per day.

Once at the mouth at Cape Henlopen, the flow continues into the waters
of the continental shelf, but because of the action of the Earth’s rotation
(Coriolis force) it turns right and continues to follow the coast of Delmarva
sometimes as far south as the mouth of Chesapeake Bay. We refer to this
feature as the Delaware Coastal Current. If it originally was transporting
oil at the mouth of Delaware Bay, it will likely distribute that oil while
gliding along the coast. If southwesterly winds blow, however, sea level falls
at the coast in connection with offshore movement of the surface water. This
transports oil and water offshore with rapid mixing with coastal seawater.

Our group has applied a mathematical numerical model of the circulation and
mixing of the Delaware Estuary and Delaware Coastal Current that has given
good agreement when tested against field observations. We are planning to use
this model to study mixing of fresh water from the Delaware estuary with the
seawater on the continental shelf. Then we will apply the model to the
detailed transport of oil seeking to simulate a spill in the estuary. Results
from this modeling effort are expected by 2006."